It is conventional to decompose water and collect hydrogen and oxygen by irradiating a semiconductor material functioning as a photocatalyst with light.
For example, Patent Literature 1 discloses a method in which a n-type semiconductor electrode and a counter electrode are disposed in an electrolyte solution, and the surface of the n-type semiconductor electrode is irradiated with light to collect hydrogen and oxygen from the surfaces of the two electrodes. Patent Literature 1 describes using a TiO2 electrode, a ZnO electrode, a CdS electrode or the like as the n-type semiconductor electrode.
Patent Literature 2 discloses a gas generator including a metal electrode and a nitride semiconductor electrode that are connected together, the two electrodes being placed in a solvent. A nitride of a Group 13 element such as indium, gallium, or aluminum is used for the nitride semiconductor electrode.
Such conventional semiconductor electrodes have a problem of low hydrogen generation efficiency in water decomposition reaction induced by irradiation with sunlight. This is because the wavelength of light absorbable by the semiconductor materials such as TiO2 and ZnO is short; that is, these semiconductor materials can only absorb light having a wavelength of approximately 400 nm or less, so that the proportion of utilizable light in the total sunlight is very small and about 4.7% in the case of TiO2. Furthermore, considering a loss of absorbed light due to a theoretical heat loss, the utilization efficiency of sunlight is about 1.7%.
TaON, Ta3N5, and Ag3VO4 have been reported as semiconductor materials that can absorb longer-wavelength visible light. However, even for these semiconductor materials, the wavelength of absorbable light is at most about 500 to 600 nm. In the case of TaON capable of absorbing light having a wavelength of 500 nm or less, the proportion of utilizable light in the total sunlight is about 19%. However, considering a theoretical heat loss, the utilization efficiency is no more than about 8%.
Meanwhile, Patent Literature 3 has recently reported that LaTaON2 is capable of absorbing visible light having a wavelength of up to 650 nm. This means that LaTaON2 is capable of absorbing the longest wavelength light among the semiconductor materials that have been hitherto reported to be capable of decomposing water. In the case of LaTaON2 capable of absorbing light having a wavelength of 650 nm or less, the proportion of utilizable light in the total sunlight is about 41%. However, considering a theoretical heat loss, the utilization efficiency is no more than about 20%.
Compound semiconductor materials containing Se, Te, or the like, and particular sulfides (such as CdS, ZnS, Ga2S2, In2S3, ZnIn2S4, ZnTe, ZnSe, CuAlSe2, and CuInS2), are also capable of absorbing light having a relatively long wavelength. However, these materials are poor in stability in water and are impractical for water decomposition reaction.
Patent Literature 4 discloses using a Group 5 element-containing carbonitride as an electrode active material for an oxygen-reduction electrode used as a positive electrode of a solid polymer fuel cell. However, Patent Literature 4 does not disclose the technical idea of using a Group 5 element-containing carbonitride as a semiconductor material functioning as a photocatalyst (photocatalytic material). In addition, the carbonitride of Patent Literature 4 is a mixture of a carbonitride with an oxide or the like, and is used in a different form from a photocatalytic material which is generally used in the form of a single-phase highly-crystalline material in view of quantum efficiency.